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[filmscanners] Re: What can you advise?

>We're both right I think. I believe you are saying that if you heat air at 5
>degrees C and 30% RH up to 25 degrees, the RH will drop to 9%. What I was
>trying to say, not totally successfully obviously, was that you cannot use
>relative humidity (RH) to compare the DRYING EFFECT of air at different
>temperatures. You have to use Vapour Pressure Deficit (Diffusion Pressure
>Deficit or Saturation deficiency) measured in kPa, mBar, or mm Hg.
>At the two temperatures I chose as examples (5 and 25 C) there will be a
>maximum amount of water that can be held by the air, and at those max levels
>the Relative Humidities will both be 100% and the VPD's will both be 0. If
>the water content of the air is reduced at both temps to give VPD in each
>case of 5 mm Hg (giving the same drying effect on anything containing
>water), then the RH's will now be about 30% at 5 degrees and 80% at 25
>degrees. These are the values I said were "equal" - equal in terms of drying
>effect or VPD (but not in terms of total amount of water held, which is what
>you were saying I think).
>The VPD is the critical factor, not RH directly, from the point of plant,
>animal, or fungal growth on films (getting back on-topic), as it is the
>measure of the drying effect of the air. There is a useful ref on this at
>It's a long, long time since I had to think about this!!
>Do you agree now that we're both right?
>Bob Frost.
>----- Original Message -----
>From: "Henning Wulff" <henningw@archiphoto.com>
>>  Actually, Bob, 100% relative humidity at 5 C is equal to about 29%
>>  relative humidity at 25 C.
>>  30% relative humidity at 5 C is about 9% relative humidity at 25 C.
>>  I'm an architect (and architectural photographer) with special
>>  qualifications relating to moisture control in buildings, and the
>>  various aspects of moisture as applicable to construction and
>>  building science - no specialized knowledge of mycology, other than
>  > how it relates to buildings :-).

See, I _knew_ that you knew about all that stuff

If anybody has further interest in this subject, a psychrometric
chart will explain all (or at least more than most people want to
know). This site has some explanation of it and also a downloadable


    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com

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